Remove experimental flag for shell and change a few apis

[PureWeen]

Description of Change

Remove the experimental flag, rename all the apis that are Shell.GetShell, and internalize some types

Issues Resolved

• fixes Xamarin.Forms.Shell Spec #2415

API Changes

Changed:

• Shell.ShellBackgroundColor => Shell.BackgroundColor
• Shell.ShellTitleColor=> Shell.TitleColor
• Shell.ShellDisabledColor => Shell.DisabledColor
• Shell.ShellUnselectedColor => Shell.UnselectedColor
• Shell.ShellForegroundColor => Shell.ForegroundColor

Removed:

• ShellItemCollection (made internal)
• ShellSectionCollection (made internal)
• ShellContentCollection (made internal)
• ShellSection.NavigationImpl (made internal)
• Shell.NavigationImpl (made internal)

Platforms Affected

• Core/XAML (all platforms)

Testing Procedure

• make sure shell works
• look through shell, shell section, shell item, shell content and see if there's anything else we should hide/expose

PR Checklist

• Has automated tests
• Rebased on top of the target branch at time of PR
• Changes adhere to coding standard

[samhouts]

 Issues/Gh5705.xaml.cs(26,60): error CS0117: 'ExperimentalFlags' does not contain a definition for 'ShellExperimental' [/Users/vsts/agent/2.150.3/work/1/s/Xamarin.Forms.Xaml.UnitTests/Xamarin.Forms.Xaml.UnitTests.csproj]
  Issues/Gh5706.xaml.cs(38,60): error CS0117: 'ExperimentalFlags' does not contain a definition for 'ShellExperimental' [/Users/vsts/agent/2.150.3/work/1/s/Xamarin.Forms.Xaml.UnitTests/Xamarin.Forms.Xaml.UnitTests.csproj